bug#8968: arc-mode 7z writing support

[Juri Linkov <juri@jurta.org>]

> I suggest to list in the order 7za, 7zr, 7z.  7za is also available as
> part of the p7zip-full package on Debian derived distributions (at
> least), so 7zr is only required for systems that only have the minimal
> p7zip package installed.

This is my understanding as well.  It seems this is what
we should do in arc-mode.el now to handle 7z and zip
until someone proves that it can be used for non-free formats.

BTW, error handling of the non-free RAR format should be more graceful.
Currently, trying to browse an incompatible RAR archive with unrar-free
displays an uninformative error message:

  Removing old name: no such file or directory

This message is shown by `delete-file' in `archive-rar-extract'.
Maybe it should check the presence of the extracted file
before trying to delete and display more clear message,
e.g. "Your RAR archive format is non-free".

Also I don't understand why the command recently added to dired-x.el
by http://thread.gmane.org/gmane.emacs.devel/136578
is "unrar x", but in `archive-rar-extract' it's still
"unrar-free --extract"?